The slow-moving mating dance between Sprint Corp. and T-Mobile US Inc. got a jolt Thursday, when France's Iliad SA made a buyout offer of its own for T-Mobile US, setting up a clash between two of the global telecommunications industry's brashest titans.

French telecom operator Iliad said it offered $15 billion in cash for 56.6% of T-Mobile US.

The development injected new uncertainty into a merger that already faces serious...